The present invention concerns an optical device that can be used for daytime vision and night vision and that enables the introduction of a superimposed collimated image into an observer's visual field.
There is a known way of using an optical combiner to introduce, into an observer's visual field, a superimposed collimated image that would, for example, give a pilot information on the flight parameters of his aircraft. The image-combining optical system used for this purpose is placed in front of the observer's eye and enables him to see the external view, generally by transparency through an optical unit, and to see the collimated image by means of an optical path comprising reflections within the optical unit. An optical device made in this way, when mounted in a helmet, constitutes what is called a helmet display unit. There is also a known way of carrying out a night observation with a helmet display unit comprising an optical night observation system followed by a light intensifier: the image given by the intensifier is combined with the collimated image by means of an auxiliary image-combining optical system of the semi-reflecting mirror type before reaching a main image-combining optical system that is placed in front of the pilot's eye and enables him to see the external view by transparency, as well as the collimated image and the intensifier image.
The auxiliary image-combining optical system gives great preference to the optical channel coming from the intensifier to the detriment of the optical channel proper to the collimated image. To prevent the optical image, in daytime vision, from being poorly visible owing to the attenuation due to the auxiliary image-combining optical system, and owing to the luminosity of the view of the external scene, the means enabling night vision are not mounted on the helmet when they are used. This results in assembling and dismantling operations that make it very inconvenient to change from daytime vision to night vision and vice versa.